Sync two external harddrives?
Solution 1:
rsync and maybe rdiff-backup.
On Windows - robocopy (comes with Vista).
Solution 2:
If you're in Windows, you can use Synctoy. Very easy to use IMO.
Solution 3:
This article compares FreeFileSync and SyncToy
Although they roughly perform the same functions, FreeFileSync had an edge over SyncToy in Performance, portability and overall has slightly more features. One advantage in particular of FreeFileSync over SyncToy is the ability to copy locked (ie open) files (SyncToy can’t copy locked files)